This invention relates to induction heating apparatus, particularly for heating heavy metal slabs, billets and such like whose thickness is not less than 20 mm., where in contrast to widespread practice the heating coils are energised by a polyphase electrical supply and are wound in a fashion corresponding to electric motor windings so as to produce a travelling wave magnetic field.
Generally, induction coils for heating metal billets and the like have involved the use of single phase windings which produce a pulsating magnetic field. In many cases, the windings have been fed from the three phase supply generally found in industry so as to avoid unbalanced loading but the windings are effectively single phase windings and simply produce an overall pulsating field--see for example U.S. Pat. No. 2,811,623 which illustrates the difficulties encountered to achieve uniform heating in the regions of the junctions between adjacent windings. Other single phase-type heaters are disclosed for instance in U.S. Pat. Nos. 2,747,068, 2,902,572 and 2,832,877.
Proposals have been made to overcome the non-uniform heating found in single-phase-type systems, by the use of polyphase windings to produce a travelling wave magnetic field instead of a pulsating field as in the single phase systems. The basic proposal appears to have been made in U.S. Pat. No. 2,005,901 to T. H. Long which discloses a strip or sheet heater in which strip or sheet material is heated from both sides by respective polyphase energized windings wound in slots formed in laminated core structures of iron or steel, the windings being wound in a double layer configuration. As a general rule, a typical thickness gauge range for sheet and strip metal is 0.004-0.50 inch, i.e. up to about 12.5 mm. In the Long heater therefore, the well-known skin-effect phenomenon would not be particularly significant in that the magnetic fluxes from both sides of the heater could penetrate the sheet or slab to such an extent that there would be considerable interaction between the two sets of flux lines. It can therefore be inferred from this that the two sets of polyphase windings in the Long heater must be wound in such a way that each pole produced by the windings on one side faces an opposite pole on the other side. If this were not the case, the Long heater would not be functional since thin material requires transverse magnetic flux but cannot support fluxes entering the material from both sides.